Our cerebral cortex, or pallium, is a big part of what makes us human: art, literature and science would not exist had this most fascinating part of our brain not emerged in some less intelligent ancestor in prehistoric times. But when did this occur and what were these ancestors? Unexpectedly, scientists at the European Molecular Biology Laboratory (EMBL) in Heidelberg, Germany, have now discovered a true counterpart of the cerebral cortex in an invertebrate, a marine worm.
It has long been clear that, in evolutionary terms, we share our pallium with other vertebrates, but beyond that was mystery. This is because even invertebrates that are clearly related to us — such as the fish-like amphioxus — appear to have no similar brain structures, nothing that points to a shared evolutionary past. But EMBL scientists have now found brain structures related to the vertebrate pallium in a very distant cousin — the marine ragworm Platynereis dumerilii, a relative of the earthworm — which last shared an ancestor with us around 600 million years ago.
Hormones control growth, metabolism, reproduction and many other important biological processes. In humans, and all other vertebrates, the chemical signals are produced by specialised brain centres such as the hypothalamus and secreted into the blood stream that distributes them around the body.
Researchers from the European Molecular Biology Laboratory [EMBL] now reveal that the hypothalamus and its hormones are not purely vertebrate inventions, but have their evolutionary roots in marine, worm-like ancestors. In this week’s issue of the journal Cell they report that hormone-secreting brain centres are much older than expected and likely evolved from multifunctional cells of the last common ancestor of vertebrates, flies and worms.
The last ancestor we shared with worms, which roamed the seas around 600 million years ago, may already have had a sophisticated brain that released hormones into the blood and was connected to various sensory organs. The evidence comes not from a newly found fossil but from the study of microRNAs — small RNA molecules that regulate gene expression — in animals alive today.